The present invention relates to a printing control system for a thermal printer and, more particularly, to a method and apparatus for controlling printing in a thermal printer.
Thermal printers have been popular as facsimile printers or the like, and various methods and circuit arrangements for controlling printing have been developed and used in practice.
A typical conventional thermal printer control system is a printing pulse control circuit for a thermal printing head described in U.S. Pat. No. 4,415,907. In the thermal printer in this prior art, in order to control heat generated by heating resistor elements during printing, the grade of heat accumulation in the printing head is analyzed on the basis of the number of dots of high level (to be referred to as "H" dots hereinafter) in the preceding dot line of the printing data. A printing pulse width is changed according to the analysis result, thereby preventing degradation of print quality which is caused by remaining heat.
In the technique described above for changing the printing pulse width in units of dot lines, when "H" dots appear continuously in a dot line, i.e., when the dot level in a direction perpendicular to the dot line is not changed, a nonuniform density distribution of the dots can be prevented. However, when the "H" dots in a dot line are not continuous and a distance between the adjacent "H" dots is increased, a nonuniform density distribution undesirably occurs.
The conventional problem will be described with reference to Table 1 below.
TABLE 1 ______________________________________ First Dot Line Second Dot Line Third Dot Line ______________________________________ Dot 1 .cndot. .cndot. .cndot. Dot 2 .cndot. .cndot. .cndot. Dot 3 .cndot. .cndot. .circle. Dot 4 .cndot. .cndot. .circle. Dot 5 .circle. .cndot. .circle. Dot 6 .circle. .cndot. .circle. Dot 7 .circle. .circle. .circle. Dot 8 .circle. .circle. .cndot. ______________________________________
One dot line consists of Dot 1 to Dot 8 which correspond to the heating resister elements, and a solid dot represents an "H" dot, and a hollow dot represents a dot of low level ("L" dot). Propositions (1) to (3) are assumed as follows:
(1) If six "H" dots or more are present in one dot line, the grade of heat accumulation is high and the printing pulse width of the next dot line is set to be a short width WS;
(2) If three to five "H" dots are present in one dot line, the grade of heat accumulation is moderate and the printing pulse width of the next dot line is set to be an intermediate width WM; and
(3) If two "H" dots or less are present in one dot line, the grade of heat accumulation is low and the printing pulse width of the next dot line is set to be a long width WL.
Under these propositions, assume that first, second, and third dot lines having 4 , 6, and 3 "H" dots are sequentially printed. Since there is no dot line preceding the first dot line, the long width WL as the printing pulse width is selected for the first dot line. Since the preceding dot line, i.e., the first dot line, of the second dot line has four "H" dots, the intermediate width WM as the printing pulse width is selected for the second dot line. Since the preceding dot line, i.e., the second dot line, of the third dot line has six "H" dots, the short width WS as the printing pulse width WS is selected for the third dot line.
The printing densities of the columns DOT1 and DOT 2 having all "H" dots are uniform, respectively, due to effects obtained by selection of pulse widths taken in consideration of the grade of heat accumulation. However, on the column DOT8, the first and second dot lines do not have printing data, i.e., "L" dots and the grade of heat accumulation is substantially low, the printing density of "H" dot for the third dot line is low because the pulse width for the third dot line is the short width WS.
In the conventional case described above, since the printing pulse width, that is, the energization time of the heating resistor elements is changed in units of dot lines in accordance with the grade of heat accumulation of the recording head as a whole, it is impossible to energize the respective heating resistor elements in accordance with different grades of heat accumulation in units of heating resistor elements.